This invention relates to the correcting of pointing error for instrumentation including antennas and other sensors carried by spacecraft encircling the earth and, more particularly, to a redirection of an instrument relative to the spacecraft to compensate for transient changes in spacecraft orientation.
Spacecraft encircling the earth in the manner of satellites may be used for observation and communication. In the case of an observation satellite, the satellite may carry photographic sensors observing cloud formation and other geographic subject matter, by way of example. Communication satellites may employ microwave antennas oriented for transmitting and/or receiving beams of electromagnetic radiation for communicating signals between the spacecraft and one or more earth stations. In both the cases of the observation satellite and the communication satellite, as well as for other spacecraft missions, it is important to maintain accurate orientation of the instrument to insure that the line of sight is pointing in a desired direction.
By way of example in the practice of such satellite missions, one, may consider a communication system employing a spacecraft encircling the earth. An antenna carried by the spacecraft for communication with an earth station may have a beam configuration which is, by way of example, generally circular with a width of 1 degree or, by way of further example, which is generally rectangular with width dimensions of 2 degrees by 0.5 degrees. With such dimensions of beam configuration, a pointing error of 0.1 degrees, by way of example, could provide a significant degradation in operation of a communications link provided by the antenna. One method of control of the orientation of an electromagnetic beam transmitted by a communications antenna is known as autotrack, and employs a receiving beam at the same antenna to view a signal transmitted by a station on the earth. Both the antenna and microwave circuitry connected to the antenna are modified by the inclusion of additional components for the detection of antenna beam pointing error, similar to that of a monopulse radar, so that antenna beam pointing error can be obtained by examination of the up-link signal received from the ground station. Information about the pointing error can then be employed by mechanical or electronic beam steering apparatus to correct the antenna beam orientation.
There are various sources of error in the orientation of the antenna (or other instrument) carried by the spacecraft, ranging from inaccuracies in the orientation of the spacecraft to dimensional changes in an antenna mount resulting from thermal expansion due to exposure to sunlight. In order to compensate for such inaccuracies, to provide for desired orientation of spacecraft instrumentation, various systems have been proposed such as, by way of example, a pointing compensation system for spacecraft instruments disclosed in Plescia et al, U.S. Pat. No. 4,687,161. Such a system compensates the instrument pointing for any disturbances by on-board motions known a priori, but does not measure the pointing errors induced by the disturbances.
Consideration is given also to short term or transient departures of spacecraft orientation from a desired orientation. Spacecraft employ thrusters and momentum wheels for correction of spacecraft orientation. A gradual reorientation of a spacecraft can be accomplished by use of one or more of the momentum wheels, while excessive departure from a desired orientation can be corrected rapidly by the firing of one or more thrusters of the spacecraft. Typically, in the control of spacecraft orientation, there may well be a hand-off between the thruster control to momentum wheel control. A firing of the thrusters can correct the spacecraft orientation within a fraction of a minute while use of the momentum wheels may employ an interval of 10-15 minutes for adjustment of the spacecraft orientation relative to the earth. Also, during the use of the thrusters, and during a hand-off between the thrusters to the momentum wheels, there is a relatively rapid change in the orientation of the spacecraft as well as in the various instruments, including antennas and photographic cameras carried by the spacecraft. Such a rapid perturbation, even if relatively small, can produce a significant and noticeable defect in the signal strength of a communication link in the situation wherein the perturbation is greater than approximately the aforementioned pointing error of 0.1 degrees.
A problem arises in that existing orientation systems and methodologies may not provide an adequate speed of response to compensate for such transient behavior of the spacecraft orientation. Even if adequate speed of response is provided, as can be accomplished with the aforementioned autotrack technology, there is a significant increase in the complexity, expense, and amount of microwave equipment which must be added to a communication system.